A receiving circuit of a signal transfer device such as a photocoupler, a magnetically-coupled transmitter of a digital isolator, an electric field coupled transmitting circuit of a capacitor, and etc, reproduces an original signal and a clock signal from an input modulated signal. A signal modulation method that uses a PWM (Pulse Width Modulation) signal is one of various modulation methods of signals. A PWM signal is a signal in which digital data that has undergone pulse width modulation is superimposed onto a clock signal. Then, a receiving circuit of the PWM method generates the output by separating the clock signal and the digital data from the input signal. Therefore, the receiving circuit includes a delay locked loop (DLL) circuit and a demodulation circuit. The DLL circuit detects the clock signal from the input signal and generates multi-phase clock signals that are delayed by a prescribed phase from the clock signal. The demodulation circuit reads the pulse width synchronously with the multi-phase clock signals and reproduces the digital data from this pulse width.
The DLL circuit generates a reference clock signal and the multi-phase clock signals based on the input signal at the same moment. Each of the multi-phase clock signals is delayed from the reference clock signal by a prescribed amount of time. The DLL circuit compares the phase of the multi-phase clock signal that is delayed by one clock and the phase of the reference clock signal that is delayed by one clock. And, The DLL circuit synchronizes the multi-phase clock signals with a reference clock signal by controlling the delay amount of the multi-phase clock signals based on this comparison result. However, such a DLL circuit is problematic in that stable operations are difficult in the initial state in which the input signal is not input.